Wide angle solar concentrator

ABSTRACT

A non-imaging optical collecting and concentrating apparatus for use in i.e., optical communications, passive lighting, and solar power applications that is relatively immune from optical incidence angle(s) and therefore does not need to track the movement of the sun to efficiently collect and concentrate optical energy is described. The apparatus includes a tubular support structure having a source-facing entrance and an energy-outputting exit. An interior surface of the structure includes a scattering, reflecting and/or diffractive medium to direct incident energy toward an exit of the tubular structure, such that the rays exiting the tube are more collimated and substantially more parallel to the axis of the tube. The collimated beam is then focused or directed by a lens or similar optical element toward a point where the energy may be collected by a detector, optical fiber, or other collection means.

FIELD OF THE INVENTION

This invention relates generally to the fields of optics, solar energy,and lighting and in particular to an apparatus that efficiently collectsand concentrates incident optical energy over a wide range of incidentangles.

BACKGROUND OF THE INVENTION

The efficient collection, concentration and distribution of solar energyremain some of the most significant, yet persistent problems ofcontemporary society. Its importance cannot be overstated. As fossilfuels continue to dwindle in supply and contribute to undesirableenvironmental effects, the importance of solar energy will onlyincrease. Efforts to realize the potential(s) of solar energy—and inparticular efforts directed toward the efficient collection andconcentration of solar energy—are therefore of great significance.

The prior art has produced a variety of solar energy collectors andconcentrators having a solar energy receiver upon which solar energy tobe collected is directed. Most of these designs feature an imagingconfiguration, where an image of the sun is produced on the receiver byan optical instrument. Such a configuration will either allow the imageof the sun to move relative to the receiver as the sun moves across thesky or will require the collector-receiver combination to track the sunduring its daily motion.

A portion of the prior art has also been devoted to so-called“non-imaging” methods, where sunlight is collected and directed at areceiver in a way that does not produce an image of the sun on thereceiver, but instead merely directs the sunlight in a random spatialpattern to the surface of the receiver. These methods typically arelimited by the range of acceptance angle of the optical system (i.e.they cannot collect over a wide range of incident solar angles), or arenot truly non-imaging (i.e. they produce a poor image which still moveson the receiver as the sun moves relative to the collector).

For example, one type of imaging solar collector is the familiarparabolic mirror which directs radiant energy incident thereon to aparticular point or focus. Examples of this type of apparatus includeU.S. Pat. No. 6,244,264 issued to R. Winston on Jun. 12, 2001, whichdescribes a single-axis parabolic reflector which can be used toconcentrate sunlight onto a long pipe or heating element. Thisconfiguration is a variant of earlier art such as the Solar RadiationCollector and Concentrator made from metallic aligned curved reflectorswhich are used to channel solar radiation to heat a cylindrical tube,described by V. J. Hockman in U.S. Pat. No. 3,964,464 (Jun. 22, 1976). Alater variant is the combination of lens and reflector troughs asdescribed by Habraken et al in U.S. Pat. No. 6,903,261 (Jun. 7, 2005).In these configurations, the reflectors described are aligned in ageneral east-west orientation so that concentrated solar radiation movesalong the tube during the day and heat is captured without diurnaltracking mechanisms, though the image of the sun moves along the lengthof the cylindrical tube. A symmetrical conical reflective concentratorwas described by Clegg in U.S. Pat. No. 4,325,612 on Apr. 20, 1982,while a large, multi-element parabolic reflector was disclosed byDietrich et al in U.S. Pat. No. 4,583,520, granted Apr. 22, 1986, and anexample of an ellipsoidal reflector concentrator can by found in U.S.Pat. No. 4,665,895, issued to Meier on May 19, 1987. Though thesedevices concentrate sunlight symmetrically in all directions, they mustbe repositioned throughout the day in order to track the sun.

Among the quasi-non-imaging solar concentrators are designs such ascurved Fresnel lenses and gradient-index (GRIN) lenses. Fresnel lensdesigns have been described in the patents of O'Neill, such as U.S. Pat.No. 4,069,812 (Jan. 24, 1978), U.S. Pat. No. 4,545,366 (Oct. 8, 1985),and U.S. Pat. No. 6,111,190 (Aug. 29, 2000). GRIN lens designs can befound in the patents of Dempewolf (U.S. Pat. No. 5,936,777, Aug. 10,1999) and Ortabasi (U.S. Pat. No. 6,057,505, May 2, 2000, and U.S. Pat.No. 6,252,155 B1, Jun. 26, 2001). For reasons described further below,the Fresnel lens and the GRIN lens must be considered imaging opticaldevices, so that a modified system based on these devices can be at besta quasi-non-imaging system, where poor solar images will move across thereceiver surface as the sun moves.

The fundamental physical constraint on these types of optical collectionsystems is the conservation of optical throughput, known from theso-called Lagrange invariant of geometric optics, which can be derivedfrom first principles. In mathematical terms, the conservation ofoptical path between two media C₁ and C₂ with boundary K is governed by$\begin{matrix}{{{{\int_{C_{1}}{n_{1}{s_{1} \cdot \quad{\mathbb{d}r}}}} + {\int_{C_{2}}{n_{2}{s_{2} \cdot \quad{\mathbb{d}r}}}} + {\int_{K}{\left( {{n_{2}s_{2}} - {n_{1}s_{1}}} \right) \cdot \quad{\mathbb{d}r}}}} = 0},} & (1)\end{matrix}$where n is the refractive index, and s is the ray vector. Thethroughput, or the product of angular acceptance and optical aperture,in a non-diffractive optical system is limited by the component with thesmallest throughput, so that $\begin{matrix}{{{\int_{C_{1}}{n_{1}{s_{1} \cdot \quad{\mathbb{d}r}}}} + {\int_{C_{2}}{n_{2}{s_{2} \cdot \quad{\mathbb{d}r}}}}} = 0.} & (2)\end{matrix}$This formulation is equivalent to the so-called Liouville form ofnon-imaging optics, wherein conservation of refractive and reflectivesystems is often expressed asn ₁ d ₁ sin α=n ₂ d ₂ sin β,  (3)where n₁ and n₂ are the refractive indices of the media on either sideof the system, d₁ and d₂ are the entrance and exit aperture widths ofthe system, respectively, and α and β are the angles over which theinput and output beams are distributed. This derivation is based on theLiouville theorem, which applies to conformal transformations betweenthree-dimensional spaces. Reflectors, lenses, Fresnel lenses, andsimilar optical instruments are all limited by this constraint.

Of critical importance in Eq. 1 is the surface K, which in refractiveand reflective optics cannot alter the wavevector ns. In diffractiveoptics, the surface K can cause discontinuity in ns, thereby allowing adifferent conservation relationship. Diffractive optics provide the onlymeans by which this constraint may be relaxed to allow larger angles andareas to be converted to smaller angles and areas, or a larger modedistribution to be condensed into a smaller distribution of degeneratemodes.

Holographic or diffractive devices have been explored in the prior art,but always in planar configuration. Such examples include the patents ofAfian et al (U.S. Pat No. 4,691,994, Sep. 8, 1987, and U.S. Pat. No.4,863,224, Sep. 5, 1989), wherein a planar hologram is coupled to aprism to guide incident sunlight by both diffraction and total internalreflection. Riccobono et al (U.S. Pat No. 5,517,339, May 14, 1996)disclose a means for exposing planar transmission holograms for solarcollection and the use (U.S. Pat. No. 5,491,569, Feb. 13, 1996) ofplanar holograms as window coverings to diffuse light into a room. Amore recent invention is that of Rosenberg, (U.S. Pat. No. 6,274,860,Aug. 14, 2001) wherein an optical radiation concentrating devicecomprises a holographic planar concentrator including a planar, highlytransparent plate and at least one multiplexed holographic opticalsurface mounted on a surface thereof. Solar collector devices can bemounted at the edges of the plate, or on the back surface of the platewhere gaps in the diffractive surface are made.

While the devices of Afian et al and Rosenberg do make use of multiplediffraction events to steer a light ray, they are limited to planarformats and rely on partial transmission of the optical radiationthrough the hologram. Afian et al in particular limit their inventionsto volume or three-dimensional transmission holograms. Rosenberg limitshis invention to a planar device with a highly transmitting platebetween sandwiched holograms. In the present invention, I disclose aconcentrator which does not rely on transmission through a hologram inorder to access the guided region where the light will be concentrated.

Though a great deal of prior art exists in this area, there exists acontinuing need for optical collecting and concentrating structuresproviding high efficiency, while eliminating the need to track thesource of the optical energy. Such structures would represent asignificant advance in the art. The present invention represents afundamental departure from prior art at the level of basic physicalprinciples as well as structural design of the system.

SUMMARY OF THE INVENTION

I have developed, in accordance with the principles of the invention, anoptical collecting and concentrating apparatus for use in, i.e., passivelighting, solar power, and optical communications applications. In sharpcontrast to prior art devices, my inventive collector and concentratoris a non-imaging, non-planar, high acceptance angle device. Consequentlyit is relatively immune from solar (or other optical source) incidenceangles and therefore does not need to track the movement of the sun toefficiently collect and concentrate solar energy.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be realizedby reference to the accompanying drawing in which:

FIG. 1 shows a perspective view of an optical collection andconcentration system constructed according to the teachings of thepresent invention, using a diffractive tube and collecting lens;

FIG. 2 shows the principle of operation of the device as the beam solarand diffuse solar illumination change throughout the day;

FIG. 3 shows the convention for positive and negative diffraction usedherein;

FIG. 4 shows a variant of the basic invention, where a flared profile isused at the output end of the tube to achieve higher collimation of theinput with a shorter tube;

FIG. 5 shows a variant of the tube design using a tapered waveguide tocouple light from the tube; and

FIG. 6 illustrates the use of a diffusing element to produce moreuniform illumination.

DETAILED DESCRIPTION

FIG. 1 shows a perspective view of a passive optical collection systemconstructed according to the present invention. More specificallycollector tube 10 includes an optically diffractive medium 12 disposedon the inner surface thereof. This medium is designed to partiallyscatter, reflect, or diffract light of various wavelengths at an anglegreater than the incident angle at which the light strikes the surface.The tube may be hollow or filled with a transparent or partiallytransparent medium. The net effect is that the output of the tube at aclear aperture 14 opposite the opening 16 where sunlight or otheroptical radiation is incident, is a beam 18 consisting of light rayswhich are substantially oriented in a direction parallel to the centralaxis 20 of tube 10. This beam thereafter impinges on a lens or similaroptical element 22, which is designed so as to focus the collimated ornearly collimated beam 18 onto an optical receiver or collecting device24. The lens element 22 may be a conventional lens or relatedinstrument, including but not limited to Fresnel lenses, reflectors, anddiffractive optical elements. The collection device may be an opticaldetector, solar cell, optical fiber, or similar type of collecting,conducting, or converting instrument.

As described above, the interior of the tube 10 is a diffractive medium12, such that rays 26 striking the interior of the tube at an angle αwith respect to the surface normal 28 are at least partially diffractedat a higher angle β. This diffractive effect is accumulated along thelength of the tube, so that the total angular spread Ω_(in) of rays 26entering the tube is greater than the angular spread Ω_(out) of rays 30exiting the tube. In optical terms, the diffractive interior of the tubetranslates a larger input numerical aperture to a smaller outputnumerical aperture.

As depicted in FIG. 2, the net result is that both beam (direct) anddiffuse solar illumination will be collected with very high efficiencyby a fixed collector/concentrator device. As the incident angle of thedirect beam solar illumination (BSI) 40 changes throughout the day, thediffractive tube 10 will collimate the beam so that the output 42 isfocused by the lens 22 to essentially the same collection point 24.

It will be apparent to skilled technicians that the diffraction grating14 must be optimized to reduce negative diffraction, or diffraction ofrays 60 in a direction proximal of the specular ray 62 with respect tothe incident ray 64, as shown in FIG. 3. Equivalently, the desireddiffractive effect is a positive one, where diffracted rays 66 aredirected along an angle greater than the specularly reflected ray 62.Proper grating design to maximize positive diffraction will in manycases also have the effect of directing scattered light substantiallymore toward the output of the tube rather than the input.

Several grating design variations may also be used to optimize themultiple diffraction effect. In particular, the angular distribution ofrays 24 striking the inner surface of the tube 10 near the entrance 12will be slightly greater than the angular distribution of rays strikingthe inner surface of the tube farther down its length, due to thediffractive effect. In particular, it can be anticipated that the numberof rays which strike the surface at near normal incidence will besuccessively diminished at points closer to the output of the tube 10.This means that while the grating must be designed to diffractefficiently over a wide range of angles, including near-normalincidence, at the entrance 12 of the tube 10, it can be designed formuch higher efficiency at glancing incidence farther down the tube,closer to the exit 16.

Likewise, the length and width of the tube can be optimized for givenmaterials and geometries. As is known from the technology of hollowwaveguides, longer tubes will result in greater interaction of the lightwith the sides of the tube, or a greater number of reflections ordiffractions and thus greater loss. At the same time, the multiplediffraction effect will require a certain number of diffraction eventsin order to confine a given percentage of incident beams into a cone ofa given output angle (NA).

Several variations of this basic concept are also encompassed within thepresent invention, including taper profiles for the basic tubular shape,which may be parabolic, hyperbolic, exponential, or a general powerseries function. As illustrated in FIG. 4, some advantage may be gainedby using a tube 80 with a flare 82 (e.g. of a axial profile described byan exponential function) at the output end. In this case, rays 84impinging on the distal end of the tube 80 at angles greater than acertain minimum determined by the geometry of the flare 82 will bereflected or diffracted by the flare 82 at an angle more parallel to thetube axis 86 than such rays would be reflected or diffracted by asimilar unflared or straight-sided tube. The effect of a flare 82 willthus be to more efficiently collimate the rays exiting the tube 80, atthe expense of the width 88 of the exiting beam. Other advantageousvariations may include a similar flare at the input of the tube.

Another variation is the use of a tapered waveguide 100 concentric to aconcentrating tube 10, as shown in FIG. 5. The waveguiding properties ofa tapered structure are known from basic optics; light rays 102 strikingthe taper at an angle α_(R) with respect to the surface normal 104 willbe totally internally reflected at the opposite side of the taperprovided that α_(R) meets the condition that $\begin{matrix}{{{\sin\quad\alpha_{R}} \geq {n_{1}{\sin\left( {{\sin^{- 1}\frac{1}{n_{1}}} - {2\theta}} \right)}}},} & (4)\end{matrix}$where n₁ is the index of refraction and θ the opening half angle of thetaper 100. It is important to note that this angle is measured relativeto the surface normal 104 of the taper on the side from which the ray isincident; relative to the axis of the tube 106, the angle is$\begin{matrix}{\alpha_{z} = {\frac{\pi}{2} - \alpha - {\theta.}}} & (5)\end{matrix}$For rays 108 incident at angles α shallower than α_(R), the angle β atwhich the ray 108 exits the opposite side of the taper will be given by$\begin{matrix}{{\sin\quad\beta} = {n_{1}{{\sin\left( {{\sin^{- 1}\frac{\sin\quad\alpha}{n_{1}}} + {2\theta}} \right)}.}}} & (6)\end{matrix}$

As a numerical example, for a glass taper with n₁=1.45 and θ=5 degrees,light incident at angles α_(z) less than about 31 degrees (α greaterthan roughly 54 degrees) will be totally internally reflected in thetaper 100. At higher incident angles α_(z), light will transmit throughthe taper, but will exit at a much shallower angle β_(z). In the sametaper as described above, a ray incident at α_(z)=32 degrees will emergefrom the opposite side of the taper at β_(z)≈9.7 degrees. The taperedwaveguide may thus be used to both capture and guide light with lowerloss than a metal clad waveguide (such as the tube itself) and may beused to ameliorate the angular translation effect of the grating tube100. The taper may be terminated with a lens to focus the light, or maycontinue past the end of the tube to guide the light into anotheroptical apparatus. Other modifications to taper 100 such asanti-reflection coatings, core-clad structures (as found in opticalfibers for communications), and varying blunt, flat, convex, or concaveends on the taper may also be used.

A further modification of my invention uses a diffusing optic 120 tomore evenly distribute the light incident at the collection point 22, asshown in FIG. 6. Since the most common distributions of light from adevice such as the tubular structure described above into a circularaperture are typically biased toward the outer portions of the circle,such a diffusing optic 120 may be designed with radially increasingscattering or diffraction toward its outer circumference. Thus, at highsolar incident angles (e.g. early or late day), rays 122 which aredirected toward the outer portion of the diffuser 120 will be partiallyscattered toward the center of the collection point 22. At mid-day, orlower solar incidence angle, the broader distribution of rays 124 willresult in less preferential scattering from the center of the diffuser120. Additionally, elliptical bias may be introduced to compensate foreast-west motion of the solar disc.

At this point, while I have discussed and described my invention usingsome specific examples, those skilled in the art will recognize that myteachings are not so limited. Accordingly, my invention should be onlylimited by the scope of the claims attached hereto.

1. An apparatus for collecting solar or other optical radiationcomprising: a tubular structure, defining an interior surface; and adiffracting medium, disposed on the interior surface of the tubularstructure; such that light rays striking the interior of the tubularstructure are directed to a common point, said common point beingsubstantially an exit aperture of the tubular structure, along ray pathswhich are substantially more aligned with the axis of the tube than thepaths along which the rays enter the tube; a lens or functionalequivalent optic, located at a point past the exit aperture of the tube,relative to the direction of propagation of rays; such that the raysexiting the tube will be directed by the lens to a collection point,such collection point being essentially the focal point of the lens orsimilar optic.
 2. The apparatus of claim 1, where the diffractivesurface of the tube is made from a surface relief grating.
 3. Theoptical apparatus of claim 2, wherein the surface relief grating isdesigned to diffract principally in a direction away from the specularreflection from the surface.
 4. The optical apparatus of claim 3, wherethe surface grating uses any combination or superposition of triangular,sinusoidal, step width, or step height variations to achieve the desireddiffraction characteristic.
 5. The optical apparatus of claim 1, wherethe diffractive surface of the tube is made from a volume hologram orother periodic refractive index structure.
 6. The apparatus of claim 1,where the diffractive surface of the tube is made from a photonicbandgap, moth-eye, or other subwavelength, periodic, or quasi-periodic,diffractive or preferentially scattering structure.
 7. The opticalapparatus of claim 1, where the tubular structure is designed topreferentially reflect or diffract light through geometric variationssuch as flared ends, tapered ends, or curved sides.
 8. The opticalapparatus of claim 1, where the lens or focusing optic is replaced orused in conjunction with a transparent tapered structure located insidethe tube to extract or guide light from inside the tube.
 9. The opticalapparatus of claim 1, where a diffusing element, diffraction grating, orsimilar device is used to more evenly distribute light over thecollection area.
 10. A method of collecting solar or other opticalenergy comprising the steps of: receiving the optical energy on asubstantially tubular structure having a diffractive surface forreceiving the optical energy; directing the optical energy byscattering, reflecting, coherently reflecting, diffracting, or anycombination thereof, to a common point which is substantially an exitpoint of the tubular structure, such that the rays exiting the tubularstructure are substantially more parallel to the axis of the tube thanthe rays entering it; focusing or otherwise directing the rays exitingthe tubular structure using a lens or similar optical instrument to acollecting point, said collecting point being essentially a focal pointof the lens; collecting the optical energy into a collector positionedat the collecting point.
 11. The method of claim 10, where thediffractive surface of the tubular structure is made from a surfacerelief grating.
 12. The method of claim 11, wherein the surface reliefgrating is designed to diffract principally in a direction away from thespecular reflection from the surface.
 13. The method of claim 12, wherethe surface grating uses any combination or superposition of triangular,sinusoidal, step width, or step height variations to achieve the desireddiffraction characteristic.
 14. The method of claim 10, where thediffractive surface of the tube is made from a volume hologram or otherperiodic refractive index structure.
 15. The method of claim 10, wherethe diffractive surface of the tube is made from a photonic bandgap,moth-eye, or other subwavelength, periodic, or quasi-periodic,diffractive or preferentially scattering structure.
 16. The method ofclaim 10, where the tubular structure is designed to preferentiallyreflect or diffract light through geometric variations such as flaredends, tapered ends, or curved sides.
 17. The method of claim 10, wherethe diffractive surface of the tube is made from a volume hologram orother periodic refractive index structure.
 18. The method of claim 10,where the lens or focusing optic is replaced or used in conjunction witha transparent tapered structure located inside the tube to extract orguide light from inside the tube.
 19. The method of claim 10, where adiffusing element, diffraction grating, or similar device is used tomore evenly distribute light over the collection area.
 20. An opticalcollector/concentrator comprising: a curved, tubular means forsupporting a scattering, reflective, or diffractive surface or anycombination thereof wherein said curved supporting means defines aninterior surface; and a means for preferentially directing light rays,disposed upon the supporting means of the tubular support structure;such that light rays striking the interior surface of the tubularsupport means are directed to a common point, said common point beingsubstantially an exit aperture of the tubular support means; a means fordirecting the rays exiting the tubular support means to a collectionpoint, said collection point being essentially the focal point of thefocusing or directing means; such that optical energy directed to thecollection point may be collected by a collection device or other meansfor collecting the optical energy.